U.S. Pat. No. 2,681,294 discloses a vacuum method for stabilizing the coating bead for direct extrusion and slide types of metered coating systems. Such stabilization enhances the coating capability of these systems. However, these coating systems lack sufficient overall capability to provide the thin wet layers, even at very low liquid viscosities, required for some coated products.
U.S. Pat. No. 2,761,791 teaches using various forms of extrusion and slide coaters to bead-coat multiple liquids simultaneously in a distinct layer relationship onto a moving web. However, these coating systems lack sufficient overall performance to maintain the desired multiple wet layer thickness at the needed web speeds and coating gaps, for some coated products. U.S. Pat. No. 5,256,357 discloses a multiple layer coating die with an underbite in one of the slot edges. Underbite in one of the two edges improves the coating situation in some cases.
U.S. Pat. No. 4,445,458 discloses an extrusion type bead-coating die with a beveled draw-down surface to impose a boundary force on the downstream side of the coating bead and to reduce the amount of vacuum necessary to maintain the bead. Reduction of the vacuum minimizes chatter defects and coating streaks. To improve coating quality, the obtuse angle of the beveled surface with respect to the slot axis, and the position along the slot axis of the bevel toward the moving web (overhang) and away from the moving web (underhang) must be optimized. The optimization results in the high quality needed for coating photosensitive emulsions. However, the thin-layer performance capability needed for some coated products is lacking.
U.S. Pat. No. 3,413,143 discloses a two slot die with excess coating liquid pumped into the coating bead area through the upstream slot. Approximately half of the entering liquid is pumped out of the bead area through the downstream slot and the remainder is applied to the moving web. The excess liquid in the bead has a stabilizing effect, which improves performance without using a vacuum chamber. However, this apparatus does not provide the performance needed for some coated products, with a maximum stated gap-to-wet-thickness ratio of only 3.
U.S. Pat. No. 4,443,504 discloses a slide coating apparatus in which the angle between the slide surface and a horizontal datum plane ranges from 35.degree. to 50.degree. and the takeoff angle defined between a tangent to the coating roll and the slide surface ranges from 85.degree. to 100.degree.. Operation within these ranges provides a compromise between performance from high fluid momentum down the slide and coating uniformity from high liquid levelling force against the slide surface. However, even with a vacuum chamber, this system does not provide the performance needed for some coated products.
A common problem encountered with extrusion die coaters has been the occurrence of streaks in the coated layer, caused by dried liquid residue on the die lips near the coating bead. This is especially true for low-viscosity liquids, containing a highly-volatile solvent. One solution to this problem, described in PCT Patent Application No. WO 93/14878 involves placing fluorine-containing resin coverings on the die faces adjacent to the lip faces to prevent wetting of these surfaces by coating liquid. This reduces streaking, dripping, and edge wariness. However, the coverings extend to the bead lip edges, and result in non-precision mechanical alignment components which are easily damaged.
European Patent Application No. EP 552653 describes covering a slide coating die surface adjacent to and below the coating bead with a low energy fluorinated polyethylene surface. The covering starts 0.05-5.00 mm below the coating lip tip and extends away from the coating bead. The low-surface-energy covering is separated from the coating lip tip by a bare metal strip. This locates the bead static contact line. The low energy covering eliminates coating streaks and facilitates die cleanup. No mention is made of using this with an extrusion coating die.
FIG. 1 shows a known coating die 10 with a vacuum chamber 12 as part of a metered coating system. A coating liquid 14 is precisely supplied by a pump 16 to the die 10 for application to a moving web 18, supported by a backup roller 20. Coating liquid is supplied through a channel 22 to a manifold 24 for distribution through a slot 26 in the die and coating onto the moving web 18. As shown in FIG. 2, the coating liquid passes through the slot 26 and forms a continuous coating bead 28 between the upstream die lip 30 and the downstream die lip 32, and the web 18. Dimensions f.sub.1 and f.sub.2, the width of the lips 30, 32 commonly range from 0.25 to 0.76 mm. The vacuum chamber 12 applies a vacuum upstream of the bead to stabilize the bead. While this configuration works adequately in many situations, there is a need for a die coating method which improves the performance of known methods.